Radiation is frequently used to treat cancer tumors. For treating localized cancers such as tumors, the goal is to maximize the radiation level at the tumor and minimize radiation damage to the rest of the body. This is achieved by irradiating the tumor with a narrow beam of radiation aimed at the tumor from many different angles so as to maximize the radiation at the tumor while sparing surrounding healthy tissue.
Prior to radiation treatment, the patient will usually receive a computed tomography (CT) scan to diagnose and locate the tumor and also to provide the anatomical information necessary to develop a treatment plan. A treatment plan consists of a series of positions for the radiation therapy source relative to the patient that will produce the desired radiation distribution centered on the tumor in the patient. Each position of the radiation therapy source may have different radiation energy levels, durations, and control of the profile of the radiation therapy beam.
It is critically important that the location of the tumor be accurately known so that the planned radiation distribution can be aligned with the tumor. If the radiation distribution is not accurately aligned with the tumor, the tumor will not receive a sufficient radiation level to damage or kill the tumor and healthy organs may receive damaging levels of radiation.
A radiation treatment system may have a linear accelerator radiation source and an x-ray imaging system consisting of an x-ray source and a large-area x-ray detector. These can be attached to a rotating mechanical gantry. By rotating the gantry around the patient, many two-dimensional x-ray projection views through the patient can be obtained and a three-dimensional cone-beam CT image can be reconstructed showing the tumor and other anatomical landmarks.
The x-ray source and large-area detector can be arranged approximately at right angles to the radiation therapy beam. This is done to avoid direct radiation from the linear accelerator striking these components, which can be damaged by the high radiation levels from the linear accelerator.
A number of real-time x-ray imaging systems are known. These include fluoroscope-based systems where x-rays are projected into an object to be x-rayed and shadows caused by relatively x-ray opaque matter within the object are displayed on the fluoroscope located on the opposite side of the object from the x-ray source. Scanning x-ray tubes have been known in conjunction with the fluoroscopy art since at least the early 1950s. Moon, Amplifying and Intensifying the Fluoroscopic Image by Means of a Scanning X-ray Tube, Science, Oct. 6, 1950, pp. 389-395.
Reverse-geometry scanning beam x-ray imaging systems are also known. In such systems, an x-ray tube is employed to generate x-ray radiation. Within the x-ray tube, an electron beam is generated and focused upon a small spot on the relatively large anode (transmission target) of the tube, inducing x-ray radiation emission from that spot. The electron beam is deflected (electromagnetically or electrostatically) in a raster scan pattern over the anode. A small x-ray detector is placed at a distance from the anode of the x-ray tube. The detector typically converts x-rays which strike it into an electrical signal in proportion to the detected x-ray flux. When an object is placed between the x-ray tube and the detector, x-rays are attenuated by the object in proportion to the x-ray density of the object. While the x-ray tube is in the scanning mode, the signal from the detector is inversely proportional to the x-ray density of the object.
Examples of known reverse-geometry scanning beam x-ray systems include those described in U.S. Pat. Nos. 3,949,229; 4,032,787; 4,057,745; 4,144,457; 4,149,076; 4,196,351; 4,259,582; 4,259,583; 4,288,697; 4,321,473; 4,323,779; 4,465,540; 4,519,092; and 4,730,350.
In a typical known embodiment of a reverse-geometry scanning beam system, an output signal from the detector is applied to the z-axis (luminance) input of a video monitor. This signal modulates the brightness of the viewing screen. The x and y inputs to the video monitor are typically derived from the signal that effects deflection of the electron beam of the x-ray tube. Therefore, the luminance of a point on the viewing screen is inversely proportional to the absorption of x-rays passing from the source, through the object, to the detector.
What are needed are a radiation therapy system and an imaging system capable of producing rapid high quality images. Furthermore, the imaging system should provide low radiation imaging and be protected from the radiation source.